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A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynamic engine during the conversion of heat into work, or conversely, the efficiency of a refrigeration system in creating a temperature difference through ...
Figure 1: A Carnot cycle illustrated on a PV diagram to illustrate the work done. Figure 2: A Carnot cycle acting as a heat engine, illustrated on a temperature-entropy diagram. The cycle takes place between a hot reservoir at temperature T H and a cold reservoir at temperature T C. The vertical axis is temperature, the horizontal axis is entropy.
The Carnot cycle is a cycle composed of the totally reversible processes of isentropic compression and expansion and isothermal heat addition and rejection. The thermal efficiency of a Carnot cycle depends only on the absolute temperatures of the two reservoirs in which heat transfer takes place, and for a power cycle is:
When a Carnot cycle runs in reverse, it is called a reverse Carnot cycle. A refrigerator or heat pump that acts according to the reversed Carnot cycle is called a Carnot refrigerator or Carnot heat pump, respectively. In the first stage of this cycle, the refrigerant absorbs heat isothermally from a low-temperature source, T L, in the amount Q L.
Since a Carnot heat engine is a reversible heat engine, and all reversible heat engines operate with the same efficiency between the same reservoirs, we have the first part of Carnot's theorem: No irreversible heat engine is more efficient than a Carnot heat engine operating between the same two thermal reservoirs.
As Carnot explained, such a cycle constitutes the most efficient heat engine possible (given the temperatures of the two reservoirs), not only because of the (trivial) absence of friction, heat leakage, or other incidental wasteful processes: The main reason is that it involves no conduction of heat between parts of the engine at different ...
The Carnot cycle achieves maximum efficiency because all the heat is added to the working fluid at the maximum temperature , and removed at the minimum temperature . In contrast, in an internal combustion engine, the temperature of the fuel-air mixture in the cylinder is nowhere near its peak temperature as the fuel starts to burn, and only ...
In modern terms, Carnot's principle may be stated more precisely: The efficiency of a quasi-static or reversible Carnot cycle depends only on the temperatures of the two heat reservoirs, and is the same, whatever the working substance. A Carnot engine operated in this way is the most efficient possible heat engine using those two temperatures.